Well heeled (part 2 of 2)

Andrew Blyth and Tom Nighy review the stability and buoyancy requirements of sailing boats.

The required minimum values of STIX are shown in the table below:

STIX, developed from the well-established Royal Yachting Association STOPS numeral, has been validated against the known behaviour of a considerable number of yachts.

Although it is possible to derive a value of STIX without a righting moment curve, most designers prefer to use it as this gives the most favourable result.

Alternative requirements are given for form-stable monohulls, which are similar to those for multihulls, are described below.

Boats that are not fully decked
Any of the following three tests may be used for assessing monohulls to satisfy the Standard’s requirements for Design Categories C or D.

Knockdown Recovery Test (see Figure 3) demonstrates that the boat will recover unaided by the crew from a knockdown. The boat is pulled over until the masthead is in the water (Category C) or the mast is horizontal (Category D) and it must recover without help from the crew. This test is used for many partially decked dayboats with ballast keels, some of which may need flotation elements in order to comply. While this is a practical test, the Standard also details a calculation option, which is often used for larger boats.

Wind Stiffness Test is generally used for boats without quick-draining cockpits or with insufficient ballast to use the Knockdown Recovery Test. It shows that a boat will not fill with water when heeled to a wind of specified strength when carrying the standard sail plan defined by the builder. The righting moment at the critical heel angle may either be measured by a test, or derived by calculation, and includes the effect of one crew seated to windward (as stability is most critical with only one person on board).

This is then used to calculate the wind speed needed to heel the boat to this angle. For Category C it must exceed 11 or 13 metres/second depending on whether this test is combined with flotation or freeboard requirements respectively, the corresponding figures being 6 or 8 metres/second for Category D.
Alternatively a boat may comply with this test when reefed, provided that:

• the reefed sail area is not less than two-thirds of the standard sail plan, and
  
• the wind strength at which the boat should be reefed is given in the Owner’s Manual, and
• warning labels are displayed to give notice of when reefing is necessary.
  

Figure 4, diagram of wind stiffness test.

This alternative is appropriate for very many traditional style boats with large sail plans that are acceptable in light weather.

As illustrated in Figure 4, the test is conducted by pulling on the mast at a convenient height with a rope that is adjusted until it is parallel with twin underwater restraint lines that prevent the boat from moving sideways. The tension measured in the mast rope and the distance between the restraint lines enables the heeling moment at that angle to be calculated. The boat must not downflood, and should not exceed 45 degrees of heel. See Figure 4.

The Capsize Recovery Test is used to show that, through the efforts of the crew, the boat can be recovered from a total inversion within five minutes, after which it must float with full crew aboard for not less than five minutes. With just one person aboard after righting, it must also float so that it can be pumped or bailed out. The minimum crew weight required must be given in the owner’s manual, as well as guidance on the likelihood of capsize in normal use and the best method for recovery. Labels must be fitted warning of the risk of capsize.
This test is used for all unballasted sailing dinghies, and follows well-established capsize-recovery procedures.

Non-Habitable Multihulls
Daysailing catamarans must be able to pass the Capsize Recovery Test already described and used for sailing dinghies.

Habitable Multihulls
The main hazard for such yachts is that they can capsize in a severe gust if too much sail is carried. It is therefore vital that proper information is available to the user. Thus ISO 12217 requires that information is provided on:

• the stability hazards of this type of boat including the risk of capsize in either roll or pitch.
  
• the wind strength at which each combination of sails should be reduced.
  
• choice of sails taking into account the wind strength, its relative direction and the sea state.
  
• precautions to be taken when altering course from a following to a beam wind, since the apparent wind speed will increase significantly giving rise to a risk of being over-canvassed.
  

It is crucial that the user does not set more sail than can be carried safely, while retaining a margin for gusts.

A calculation method is provided to derive the maximum safe wind speeds for each sail plan, so that the boat will not capsize even if the wind pressure doubles in a gust. This method is currently under review because of new information about the wind-heeling characteristics of multihulls. Pitch stability, as well as roll stability, is taken into account as very wide boats can be vulnerable to pitchpoling — an end-over-end capsize.

In addition to requiring warning symbols to alert users to the importance of this information, the standard also requires that if the yacht should capsize, it must have sufficient inherent buoyancy to float inverted with an adequate margin. A calculation method is provided to show that this is so. The method requires knowledge of the weight and density of each of the materials used in building the boat, so that the total buoyant volume can be determined.

The Design Category of a multihull yacht is determined using an empirical Multihull Size Factor, which is based on the length, beam and minimum operating weight of the boat. This method attempts to quantify the vulnerability to capsize when laying-to under bare poles. Hopefully further work may enable a better method to be devised in the future.
The monohull requirements for downflooding openings and downflooding height also apply to multihulls.

Proforma Worksheets
As with non-sailing boats, the standard provides a series of standard worksheets to facilitate the assessment process. Each stage in the process is cross-referenced to the text of the Standard.

In the next issue, European Boatbuilder April 2004: We look at the requirements for cockpits, windows, hatches and doors.

For Part 1 click here

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